This invention relates to a method and means for improving an imaging system. In greater particularity it is for a method and means for incorporating the operations of resolution enhancement and apriori information utilization simultaneously in the design of an improved imaging system. In still greater particularity it is to provide for an improved method and means for improving an imaging system that is adapted to imaging in a synthetic aperture radar, or in an inverse synthetic aperture radar, a radiometer, a sonar, an electromagnetic or acoustic tomographic system or a related system in which there is an interaction between the measurements that are being taken of physical phenomenon and the phenomenon which are being observed.
Recently an analogy has become recognized which exists between delay-doppler imaging-radar systems and tomographic systems used in clinical radiology. The analogy appears to hold the possibility of improving radar imaging because the use of matched filtering for noise suppression is suggested even by initial comparisons, and, more importantly because a line of thinking is emerging by which new mathematical models for the radar-imaging problem might be formulated and solved for improving processing. These new models account for dominant effects including noise. M. Bernfeld, in his article entitled "Chirp Doppler Radar" Proceedings IEEE, Vol. 72, No. 4 pp 540-541, April 1984, made a restricted form of this observation and the restricted form also appears in a different form in the work of D. Mensa, S. Halevy, and G. Wade in their article entitled "Coherent Doppler Tomography for Microwave Imaging" Proceedings IEEE, Vol. 71, No. 2 pp 254-261, February 1983. Both of these articles draw the analogy to a tomography system wherein the data available for processing are in the form of idealized, noise-free line-integrals through the object being imaged. This type of tomography system embraces a situation that is well approximated with X-ray tomography systems because X-ray sources can be highly collimated so as to form narrow X-ray beams of high intensity that are passed through the object being imaged. Although the analogy was articulated in these two articles, there is strong reason to believe that its applicability to practical radar/sonar signals of interest is limited because the ambiguity functions normally associated with such radar/sonar signals do not approximate line distributions in mass and thus do not permit the evaluation of line integrals of the scattering function. Two additional writings dealing with frequency-stepped, chirp-signals have discussions which clarify this limitation. M. Prickett, and C. Chen in "Principles of Inverse Synthetic Aperture Radar (ISAR) Imaging," IEEE EASCON Record, pp. 340-345, September 1980 and M. Prickett and D. Wehner in "Stepped Frequency Target Imaging", Applications of Image Understanding and Spatial Processing to Radar Signals for Automatic Ship Classification Workshop, New Orleans, La., February 1979 discuss side lobe structures and other features that cause a departure from idealized line-integrals and the fact that noise can be non-negligible in some radar-imaging situations. A solution was not evident, however. These articles are included in the Appendix for a reader's convenience.
Thus, there is a continuing need in the state-of-the-art for a method and means which may permit the removal of the restriction of noise-free line-integrals so that general magnitude squared ambiguity functions can be accommodated and the recognition of the effects of noise can be developed for improved imaging. In this discussion the ambiguity function is defined as the magnitude squared of the time-frequency autocorrelation or cross correlation function.